Nanoribbons with Nonalternant Topology from Fusion of Polyazulene: Carbon Allotropes beyond Graphene

Article abstract of DOI:10.1021/jacs.9b08060

Various two-dimensional (2D) carbon allotropes with nonalternant topologies, such as pentaheptites and phagraphene, have been proposed. Predictions indicate that these metastable carbon polymorphs, which contain odd-numbered rings, possess unusual (opto)electronic properties. However, none of these materials has been achieved experimentally due to synthetic challenges. In this work, by using on-surface synthesis, nanoribbons of the nonalternant graphene allotropes, phagraphene and tetra-penta-hepta(TPH)-graphene, have been obtained by dehydrogenative C-C coupling of 2,6-polyazulene chains. These chains were formed in a preceding reaction step via on-surface Ullmann coupling of 2,6-dibromoazulene. Low-temperature scanning probe microscopies with CO-functionalized tips and density functional theory calculations have been used to elucidate their structural properties. The proposed synthesis of nonalternant carbon nanoribbons from the fusion of synthetic line-defects may pave the way for large-area preparation of novel 2D carbon allotropes.

Full text of DOI:10.1021/jacs.9b08060

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Article
Nanoribbons with Non-Alternant Topology from Fusion
of Polyazulene: Carbon Allotropes Beyond Graphene
Qitang Fan, Daniel Martin-Jimenez, Daniel Ebeling, Claudio K. Krug, Lea Brechmann, Corinna
Kohlmeyer, Gerhard Hilt, Wolfgang Hieringer, Andre Schirmeisen, and J. Michael Gottfried
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b08060 • Publication Date (Web): 16 Oct 2019
Downloaded from pubs.acs.org on October 19, 2019
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Nanoribbons with Non-Alternant Topology from Fusion of Polyazulene:
Carbon Allotropes Beyond Graphene
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Qitang Fan1‡, Daniel Martin-Jimenez2,3‡, Daniel Ebeling2,3*, Claudio K. Krug1, Lea Brechmann1, Corinna
Kohlmeyer4, Gerhard Hilt4*, Wolfgang Hieringer5*, André Schirmeisen2,3, J. Michael Gottfried1*
1Department of Chemistry, Philipps University Marburg, Hans-Meerwein-Straße 4, 35032 Marburg, Germany
2Institute of Applied Physics (IAP), Justus Liebig University Gießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany
3Center for Materials Research (LaMa), Justus Liebig University Gießen, Heinrich-Buff-Ring 16, 35392 Gießen,
Germany
4Institute of Chemistry, Carl von Ossietzky University Oldenburg, Carl-von-Ossietzky-Straße 9-11, 26111 Oldenburg,
Germany
5Theoretical Chemistry and Interdisciplinary Center for Molecular Materials (ICMM), Department of Chemistry and
Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany
ABSTRACT
Various two-dimensional (2D) carbon allotropes with non-alternant topologies, such as pentaheptites and
phagraphene, have been proposed. Predictions indicate that these metastable carbon polymorphs, which
contain odd-numbered rings, possess unusual (opto)electronic properties. However, none of these
materials has been achieved experimentally due to synthetic challenges. In this work, by using on-surface
synthesis, nanoribbons of the non-alternant graphene allotropes, phagraphene and tetra-penta-
hepta(TPH)-graphene have been obtained by dehydrogenative C-C coupling of 2,6-polyazulene chains.
These chains were formed in a preceding reaction step via on-surface Ullmann coupling of 2,6-
dibromoazulene. Low-temperature scanning probe microscopies with CO-functionalized tips and density
functional theory calculations have been used to elucidate their structural properties. The proposed
synthesis of non-alternant carbon nanoribbons from the fusion of synthetic line-defects may pave the way
for large-area preparation of novel 2D carbon allotropes.
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INTRODUCTION
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Modifying the topology of pristine graphene with atomic precision has attracted tremendous attention in
recent years, because it serves as a versatile route to novel graphene-related materials with exotic
(opto)electronic properties.1-3 Examples include graphene nanoribbons,4-8 porous graphene,9-11 and
nanographenes,12-16 which have been successfully prepared through various bottom-up approaches. The
majority of these materials are cut-outs of graphene, i.e., they are composed of sp2 carbon atoms
arranged to form benzenoid hexagonal rings. The electronic properties of these so-called alternant -
electron systems17 are determined by factors such as size, shape, and edge structure. An alternative and
not yet well-developed method to modify graphene is to change the connectivity pattern of the sp2 carbon
atoms in a non-hexagonal (non-benzenoid) manner. Theoretical considerations have predicted the
existence of numerous planar non-benzenoid carbon allotropes18-20, including pentaheptites (1, Figure
1a),21-23 haeckelites,24 T-graphene,25 octa-penta graphene,26 pentahexoctites,27 and phagraphene 2.28
These 2D carbon allotropes are expected to show dramatically different (opto)electronic properties,29,30
electron transportation,31, 32 magnetism,33 chemical reactivity34 and mechanical stability.33, 35, 36 However,
the experimental realization of these materials lags far behind the theoretical studies. This problem is
mainly caused by the enormous challenges that lie in the atomically precise control over the growth of
extended non-benzenoid carbon structures. Although non-benzenoid structural motifs, such as the Stone-
Wales (5-7-7-5) and 5-8-5 defects, can be randomly introduced into graphene by moderate electron beam
bombardment, irradiation or chemical treatment,37, 38 the periodic arrangement of these structural
elements is so far difficult to achieve.
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Very recently, the on-surface synthetic approach has demonstrated its potential for the growth of non-
benzenoid 2D carbon allotropes in a more controlled way. Through cyclodehydrogenative C-C coupling of
specially designed benzenoid organic molecules, it was possible to make nanographenes with embedded
5-7 ring pairs14, 39-41 and graphene-like carbon nanoribbons with periodically arranged 4-8-4 motifs.42, 43
The growth of polymers with periodically arranged odd-numbered polygon motifs, e.g., the 5-7 ring pairs,
is intriguing because the connectivity pattern of carbon atoms alters from alternant (hexagons) into non-
alternant.17 In contrast, the 4-8 structural motif is non-hexagonal, but remains alternant. The former case
is especially interesting, because molecular -electron systems with non-alternant topology have
drastically different electronic properties compared to their alternant counterparts.17, 44 All the above
mentioned examples of growing 5-7 motifs represent only embedded point defects. Up to now, the
embedment of a continuous one-dimensional (1D) arrangement of pentagon-heptagon (5-7) motifs is still
lacking.
Here, by employing 2,6-dibromoazulene (4, Figure 1b) as a non-alternant non-benzenoid precursor
consisting of annulated 5- and 7-membered rings, we have successfully obtained the 2,6-polyazulene
chains 5 via on-surface Ullmann coupling. The 2,6-polyazulene chain could be reckoned as 1D line defect
with fused pentagons and heptagons (5-7 motif). Subsequently, the lateral dehydrogenative coupling
(fusion) of two 2,6-polyazulene chains results in the formation of carbon nanoribbons that are rich in 5-7
defects and locally resemble the phagraphene nanoribbon 6 or the TPH-graphene nanoribbon 7. The two
different carbon nanoribbons are formed due to the different relative positions of the two fused 2,6-
polyazulene chains. These two carbon nanoribbons represent the stripes (colored) cut out from two yet
unachieved 2D carbon allotropes, phagraphene 2 and TPH-graphene 3. The structures of these two
nanoribbons are elucidated unambiguously by scanning tunneling microscopy (STM), non-contact atomic
force microscopy (AFM) in ultrahigh vacuum environment at low temperature (5.2 K) by using CO-
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functionalized tuning-fork sensors, and density functional theory (DFT) calculations.45 The synthetic
strategy used in this work may pave the way for the synthesis of other extended carbon allotropes with
non-benzenoid and non-alternant character.
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Figure 1. Reaction scheme for the synthesis of nanoribbons of non-alternant 2D carbon allotropes. (a)
Skeletons of three proposed 2D carbon allotropes, i.e., pentaheptite 1, phagraphene 2, and THP-graphene 3;
all consisting of sp2 carbon atoms. The colored stripes denote the nanoribbons achieved in this work. (b)
Ullmann C-C coupling of 2,6-dibromoazulene (2,6-DBAz) 4 into 2,6-polyazulene 5 chains and their fusion into
carbon nanoribbons rich of non-hexagonal rings, including phagraphene nanoribbon 6 and TPH-graphene
nanoribbon 7. These two carbon nanoribbons correspond to the stripes cut out from the proposed 2D carbon
allotropes in panel (a).
RESULTS
On-surface synthesis of polyazulene chains. In the first step, polyazulene chains have been prepared on
a Au(111) surface by Ullmann coupling. As shown by Figure 2a, deposition of a submonolayer of 2,6-DBAz
onto Au(111) at 300 K followed by two annealing steps at 480 K and 690 K leads to ordered domains of
chains. The detailed structure of these chains is revealed by a combination of high-resolution STM (Figure
2b) and constant-height AFM (Figure 2c) images of a section of the domain (red-framed region in Figure
2a), using a CO-functionalized tip.46 As can been seen in Figure 2b, weak protrusions are observed between
the chains, which are assigned as bromine adatoms formed by C-Br bond scission. The resulting Br…H
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hydrogen bonds are the driving force for the parallel arrangement of the polyazulene chains, similar to
previous findings for polyphenylene chains.47 The AFM image enables the visualization of the C-C bonds
in the azulene units, revealing the feature of annulated 5- and 7-membered rings, in agreement with
previous AFM images of single azulene molecules.17 Therefore, the molecular structures of these
polyazulene chains can be assigned unambiguously, as is exemplified for the left-most chain in the red
frame in Figure 2c. The corresponding chemical structure is given in Figure 2d. As can be seen, the azulene
units in this chain are not uniformly oriented. The irregularities result from the three different possible
types of C-C coupling. A uniform, regular orientation would require that the 5-membered ring of one
azulene unit binds to the 7-membered ring of its neighbor (5-7 coupling). However, 5-5 and 7-7 coupling
motifs also occur, as indicated by the labels in Figure 2d. The 7-7 coupling type is associated with
characteristic bright features, which can be explained by a nonplanar adsorption conformation due to in-
plane steric repulsion between C-H bonds of two neighboring azulene units (highlighted with red color).
Similar features have been observed previously in AFM images of twisted phenyl units or aliphatic
hydrocarbons.48-51 In contrast, for the 5-7 and 5-5 coupling motifs, the involved azulene units exhibit
almost planar adsorption configurations, indicating reduced repulsion due to the larger distances between
the C-H bonds (Figure 2c, colored blue and green). The relative yields for these three types of coupling
amount to 21.2% (5-5), 54.6% (5-7), and 24.2% (7-7) (see Figure S1 in the Supporting Information (SI) for
details). These ratios are close to the expected random distribution of 1:2:1, indicating trivial selectivity
among these three coupling types.
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Figure 2. STM and nc-AFM images of polyazulene chains on Au(111). (a,e) Overview STM images of polyazulene
chains formed after deposition of a submonolayer of 2,6-DBAz onto Au(111) at 300 K followed by successive
annealing to 480 K for 30 minutes and 690 K for 20 minutes (a), and 730 K for 30 minutes (e). (b,c) Zoom-in
STM (b) and constant-height AFM (c) images of the red-framed region in panel (a). (d) The chemical structure
of the chain marked by the red frame in panel (c) highlights the different types of C-C coupling (5-7, 5-5, and
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7-7). (f-h) Zoom-in STM (f), constant-height AFM (g) images and chemical structure (h) of the curved chain in
the red-framed region of panel (e). The methylene-indene units formed by isomerization of azulene units has
been colored red in (h). Image parameters: (a,f) U = 10 mV, I = 10 pA; (b) U = 10 mV, I = 70 pA; (e) U = 100 mV,
I = 10 pA. Tip distances for the constant-height AFM images: (c) z = 50 pm, and (f) z = 25 pm with respect to the
tunneling gap at an STM set point of I = 10 pA and U = 10 mV above the Au(111) substrate. All the STM and
AFM images were measured with a CO-functionalized tip and an oscillation amplitude of 70 pm.
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Further annealing of the sample in Figure 2a to 730 K results in the separation and occasional bending of
the polyazulene chains, as shown by the STM image in Figure 2e. In addition, a certain amount of single
chains cross-link at their terminals. These structural changes can be interpreted as follows. Annealing to
730 K desorbs the bromine adatoms from Au(111). This is evidenced by the absence of the weak
protrusions around the chains. As a result, the chains have an increased probability to encounter and react
through dehydrogenative coupling. Note that the C-H bond scission is expected to occur on the Au(111)
surface at 730 K, according to previous work.13, 52, 53 The occasional bending of the polyazulene chains is
caused by rearrangements in the carbon framework, as is demonstrated for a typical single chain with two
bends (red-framed region in Figure 2e). Its structure has been scrutinized by detailed STM (Figure 2f) and
AFM (Figure 2g) measurements and is shown in Figure 2h. The straight part of this chain consists entirely
of azulene units, whereas the bend bears a methylene-indene moiety, as marked by red color. Since
methylene-indene is an isomer of azulene, it is likely that an azulene moiety undergoes C-C bond
rearrangement (or isomerization) into the methylene-indene moiety on Au(111) already at 730 K. Besides
the methylene-indene motif, naphthalene as another isomer of azulene has occasionally been observed
as a structural motif at the corners of a single curved chain, as shown by Figures S4e-g in the SI. Related
carbon-carbon (C-C) bond rearrangements have been reported for a polycyclic aromatic hydrocarbon with
azulene moieties on a Cu(001) surface.39 Interestingly, the 5-6-5-7-5 fused-ring moieties at the two
terminals of the chain in Figure 2f show a prominent electronic state, which is not present at the azulene
moieties. This feature may be related to the antiaromatic character of the 20 π-electron system, which
may be in resonance with an open-shell diradical structure (Figure S3 in the SI, right part), similar as
reported in previous work.54-56 Charging such moieties by electron transfer from the surface (and the
surrounding π-electron system) may lead to stabilization due to increased aromatic character.
Lateral fusion of polyazulene chains into non-alternant carbon nanoribbons. At this stage (after heating
to 730 K, see Figure 2d) also pairwise lateral dehydrogenative fusion of the polyazulene chains is induced,
leading to the formation of wider nanoribbons as presented by Figures 3a and 3e showing different
sample regions. Two representative nanoribbons are selected for detailed structural analysis. The
structure of the nanoribbon in the red-framed region in Figure 3a is revealed by the zoom-in STM (Figure
3b) and the corresponding bond-resolving AFM (Figure 3c) images. As illustrated in Figure 3d, the
nanoribbon contains two chains (blue colored) that are fully fused forming polygons (red colored) in the
interspace. The major structural motif in the chains (blue) is azulene. Minority structural motifs are
naphthalene (marked by blue arrows) and methylene-indene moieties (marked by red arrows), which
originate from the isomerization of the azulene moieties in the polyazulene chains as discussed above. A
short section of the nanoribbons marked by the red bracket in Figure 3d represents the perfect lateral
fusion of two oppositely oriented, regular 2,6-polyazulene chains. As can be seen, hexagons are formed
exclusively between the two chains. Therefore, this structure can be considered as a stripe cut out from
the carbon-allotrope phagraphene proposed in previous theoretical work.28 Another nanoribbon with
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different structure is marked by the red frame in Figure 3e; its enlargement is shown in Figure 3f. The
detailed structure of this ribbon, as derived from the AFM image in Figure 3g, is displayed in Figure 3h. As
can be seen, this nanoribbon is again based on two single chains consisting of C-C bonded azulene (and
methylene-indene) moieties. Similar to the first nanoribbon (Figure 3c), the section marked by the red
bracket shows a periodic structure derived from two oppositely oriented regular 2,6-polyazulene chains.
Compared to the first case (Figure 3d), the two chains have a shifted relative position, such that a carbon
nanoribbon with four-, five-, and seven-membered rings is obtained. Thus, this nanoribbon represents a
stripe of the TPH-graphene in Figure 1a. Additional examples of these two types of non-alternant carbon
nanoribbons are shown in Figure S2.
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Figure 3. STM and nc-AFM images of carbon nanoribbons on Au(111). (a,e) Overview STM images of different
regions of the sample (same as in Figure 2d), showing wider nanoribbons with different structures. (b-d) High
resolution STM (b), constant-height AFM (c) images and chemical structure (d) of the nanoribbon in the red-
framed region in panel (a). (f-h), High resolution STM (f), constant-height AFM (g) images and chemical
structure (h) of the nanoribbon in the red-framed region in panel (e). In (d,h), the blue color marks the two
chains fused into the nanoribbon by forming polygons marked by red color. Blue and red arrows point out the
naphthalene and methylene-indene moieties, respectively, formed by isomerization of azulene moieties.
Image parameters: (a,b) U = 10 mV, I = 10 pA; (e) (f) U = 100 mV, I = 10 pA. Tip distances for the AFM images:
(c) z = 120 pm, and (g) z = 80 pm with respect to the STM set point of I = 10 pA and U = 10 mV above the Au(111)
substrate. All the STM and AFM images are measured with a CO-functionalized tip and an oscillation amplitude
of 70 pm.
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DISCUSSION
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The high-yield formation of longer phagraphene 6 and TPH-graphene 7 nanoribbons is still challenging,
mainly because of two issues: First, carbon allotropes with non-alternant structural elements may
isomerize to their corresponding alternant structures, which are usually more stable. In particular,
isomerization of the azulene units in 6 and 7 to naphthalene units is expected to be energetically favorable,
considering that naphthalene is 1.62 eV more stable than azulene.57 If C-C bond rearrangements are
kinetically possible under the reaction conditions, these transformations are likely to occur, as shown in
Figure S4. The stability of the non-alternant structures is therefore an important aspect of their on-surface
synthesis. Indeed, the present periodic DFT calculations for the electrically neutral free-standing, planar
nanoribbons (cf. Figure 4) suggest that 6 (Figure 4a) is thermodynamically less stable by 2.7 eV per C20H6
formula unit than its alternant counterpart 8 shown in Figure 4b. Both structures enclosing four-
membered rings (7 and 9, Figures 4c,d) are even more destabilized by 5.2 and 7.2 eV, respectively,
compared to the low-energy structure 8. This may be rationalized based on the high ring tension within
the four-membered rings in 7 and 9. The hypothetical naphthalene-based nanoribbon 9 also features an
exceptionally long C-C bond of 179 pm, indicating a highly strained structure.
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In the following, we discuss that the relative stabilities of the different structures are also consistent with
qualitative aromaticity considerations. The color code of the bonds in Figure 4 shows the calculated C-C
bond lengths for the two nanoribbons 6 and 7 (Figures 4a,c) and for the corresponding hypothetical
structures 8 and 9 formed by isomerization of their azulene units to naphthalene units (Figures 4b,d).
Based on the C-C bond lengths, the aromatic character of the rings can be assessed using the harmonic
oscillator model of aromaticity (HOMA).58 A higher HOMA value is generally associated with increased
aromatic stabilization.58 The HOMA values of the rings are illustrated by the different shades of orange
color in Figure 4. Comparison of the phagraphene ribbon 6 with its alternant isomer 8 reveals that the
latter has considerably higher HOMA values and thus experiences increased aromatic stabilization.
Therefore, isomerization of 6 to its alternant counterpart 8 should be energetically favored, in agreement
with the experimental observations (Figure S4) and the DFT energies (E8-6 = -2.7 eV). In contrast, the
HOMA values indicate that the TPH-graphene nanoribbon 7 is more stable than its hypothetical
isomerization product 9, again in line with the DFT results (E9-7 = +2.0 eV). Comparison of the HOMA data
for the two experimental nanoribbons 6 and 7 reveals a slight advantage in aromatic stabilization for 6, in
agreement with the difference of their DFT energies (E7-6 = +2.5 eV). According to these considerations,
7 may be more resistant to isomerization of azulene to naphthalene units than 6. Furthermore, a
preliminary theoretical analysis of the electronic structures at the DFT-GGA level suggests smaller band
gaps of the non-alternant phagraphene 6 (Figure S5a, cf. also Ref. 28) and THP-graphene 7 (Figure S5b)
nanoribbons compared to their alternant counterparts 8 (Figure S5c) and 9 (Figure S5d), respectively. This
is in analogy to the lower HOMO-LUMO gap of azulene compared to naphthalene.17
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Figure 4. DFT-calculated C-C bond lengths of planar carbon nanoribbons. (a,c) Optimized gas-phase chemical
structure of the non-alternant phagraphene nanoribbon 6 (a) and TPH-graphene nanoribbon 7 (c). (b,d) GNR
counterparts of phagraphene and TPH-graphene nanoribbons obtained by isomerization of azulene moieties
into naphthalene in these two types of nanoribbons (8 and 9). The bond lengths and harmonic oscillator model
of aromaticity (HOMA) values of each polygon are presented with gradient colors as illustrated by the color
scales at the right part of the figure. The average HOMA values are (a) 0.426, (b) 0.614, (c) 0.282, (d) -1.48.
The second challenge in the synthesis of phagraphene and TPH-graphene nanoribbons arises from the
requirements of structural homogeneity and alignment of the 2,6-polyazulene chains. These chains should
exclusively contain 5-7 connections and have opposite orientations. The first condition is demanding
considering that the polyazulene chains are formed with randomly orientated azulene units at the first
Ullmann coupling step. The second requirement is the reason why wider nanoribbons with more than two
fused chains have only rarely been observed: They require alternating orientations of the 2,6-polyazulene
chains, which is of diminishing probability with the increasing width of the ribbon.
CONCLUSION
In conclusion, carbon allotrope nanoribbons with non-alternant topologies of their -electron systems
have been achieved by the lateral dehydrogenative coupling of polyazulene chains formed in a preceding
step via surface Ullmann coupling of the 2,6-dibromoazulene precursor on Au(111). The distinctive
phagraphene nanoribbon 6 and the THP-graphene nanoribbon 7 sections are frequent structural elements
observed in these carbon allotrope nanoribbons. The different structures of these nanoribbons are
determined by the relative positions of the fused polyazulene chains. Their yields are limited by the mixed
connection types and isomerization of the azulene units in the polyazulene chains. DFT calculations
suggest that the non-alternant phagraphene and THP-graphene nanoribbons exhibit lower band gaps than
their alternant counterparts. These non-alternant carbon nanoribbons may represent promising
candidates for the search of new carbon-based quantum materials with exotic properties. Moreover, the
hierarchical coupling of non-alternant and non-benzoid conjugated rings of carbon atoms opens up
possibilities for the synthesis of novel 2D carbon allotropes.
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METHODS
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Synthesis: 2,6-Dibromoazulene was prepared by using a published procedure as described in the
Supporting Information part 3. The details of the synthesis and characterization of 2,6-dibromoazulene
are also included in the Supporting Information part 3.
9
Sample preparation: The Au(111) crystal (MaTecK, Germany) was cleaned by multiple cycles of Ar+
sputtering (E = 1.5 kV, I = 3.6 µA, p = 6 × 10−6 mbar for initial cycles and E = 0.8 kV, I = 1.1 µA, p = 3 × 10−6
mbar for final cycles) and annealing (1000 K for initial cycles and 730 K for final cycles). The 2,6-DBAz
molecules were evaporated on the Au(111) surface by using a home-built evaporation device held at room
temperature.59 To induce chemical reactions on the surface, the sample was placed in a heating station
(Scienta Omicron, Germany) where it was heated by thermal radiation of a tungsten filament. The sample
temperatures were deduced from the heating power of the filament and a thermocouple that is attached
to the heating station close to the sample plate. The heating station was previously calibrated by using a
second thermocouple that was spot-welded to the center of an empty sample plate.
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STM/AFM: STM and AFM images were obtained at 5.2 K and a pressure below 1 × 10−10 mbar with a low
temperature AFM/STM (Scienta Omicron, Germany). A Q-plus tuning fork sensor with a tungsten tip was
used. The tip was sharpened by voltage pulses and indentations into the Au(111) surface. The AFM tip
was functionalized with a CO molecule using standard procedures described in the literature.60 The
resonance frequency and quality factor of the sensor (fr = 25.866 kHz, Q = 18000-25000) was determined
by acoustic tunes with a tip-sample separation of about 10 nm. The Q-plus sensor was driven in frequency
modulation mode using an external phase-locked loop electronics (MFLI, Zürich Instruments, Switzerland)
for obtaining both STM and AFM images.51 The oscillation amplitude of the sensor was kept constant at
70 pm. AFM images were taken in constant-height and constant-current scanning modes.51
Computational details: Periodic density functional-theory calculations of free-standing carbon
nanoribbons were performed with the Vienna Ab Initio Simulation Package (VASP)61 using the PBE
functional62 and the D3(BJ) dispersion correction.63, 64 Please see the Supporting Information for further
details.
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is available free of charge on the ACS Publications
website. Precursor synthesis and characterization, additional STM and AFM images, and details of the DFT
calculations.
AUTHOR INFORMATION
Corresponding Authors
* daniel.ebeling@ap.physik.uni-giessen.de
* gerhard.hilt@uni-oldenburg.de
* wolfgang.hieringer@fau.de
* michael.gottfried@chemie.uni-marburg.de
Author Contributions
‡ Q.F. and D.M.J. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
Financial support by the Deutsche Forschungsgemeinschaft via the grants 223848855-SFB 1083, GO
1812/2-1, HI 655/18-1, EB 535/1-1, and SCHI 619/13, the Cluster of Excellence EXC 315 ‘‘Engineering of
Advanced Materials’’, and the GRK 2204 "Substitute Materials for Sustainable Energy Technologies" is
gratefully acknowledged. The RRZE of the University of Erlangen−Nürnberg is acknowledged for
computational resources. Q.T.F thanks the Alexander von Humboldt-Foundation for a Research
Fellowship for Postdoctoral Researchers.
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